Last-Modified

A web page, image or other resource will be served with a Date Header. This indicates when that response was generated by the origin server. If you send the same request you should get back a response with a new Date Header. The "Last-Modified" header is slightly different. It returns a date too, but it represents the time when the resource was last updated. If the resource is seldom modified then this date could be substantially in the past and it wont change if you request the resource again. The "Last-Modified" and "Date" headers usually have different values.

The "Last-Modified" header can be used by web browsers as part of its caching strategy to determine if a response should be stored in the cache and served up again in the future.